Isolated C=C 1640-1680cm-1 Conjugated C=C 1620-1640cm-1 Aromatic C=C w1600cm-1 Carbon-carbon Triple Bonds CC triple bonds only absorb if they are terminal,since internal alkynes have very small (almost zero)bond dipole moments. Carbon Hydrogen Bond Stretching The C-H bond stretch is subtlety affected by the hybridization of the C-H bond. The more s character a bond has,the stronger it is,and so the higher wavenumber it should absorb at. sp'is one third s,absorbs at 3000-3100cm sp is one half s,absorbs at ~3300cm Interpretation of IR Spectra Figures 12-6,12-7(SLIDES) Characteristic Absorptions of Alcohols and Amines The O-H and N-H bonds are strong and therefore stiff,so they occur at high frequencies,usually above 3000cm Typical Stretching Frequencies Amine N-H 3300cm broad and with spikes Acid O-H 3300cm'broad Alcohol O-H 3000cm broad These are almost always broad since these molecules exhibit extensive hydrogen bonding,giving rise to many differing bonds,and so many different stretching frequencies. Figures 12-8,12-9(SLIDES) Chl2 IR and MS Page9Isolated C=C 1640-1680cm-1 Conjugated C=C 1620-1640cm-1 Aromatic C=C ~1600cm-1 Carbon-carbon Triple Bonds CC triple bonds only absorb if they are terminal, since internal alkynes have very small (almost zero) bond dipole moments. Carbon Hydrogen Bond Stretching The C-H bond stretch is subtlety affected by the hybridization of the C-H bond. The more s character a bond has, the stronger it is, and so the higher wavenumber it should absorb at. sp3 is one quarter s, absorbs at 2800-3000cm-1 sp 2 is one third s, absorbs at 3000-3100cm-1 sp is one half s, absorbs at ~3300cm-1. Interpretation of IR Spectra Figures 12-6, 12-7 (SLIDES) Characteristic Absorptions of Alcohols and Amines The O-H and N-H bonds are strong and therefore stiff, so they occur at high frequencies, usually above 3000cm-1. Typical Stretching Frequencies Amine N-H 3300cm-1, broad and with spikes Acid O-H 3300cm-1, broad Alcohol O-H 3000cm-1, broad. These are almost always broad since these molecules exhibit extensive hydrogen bonding, giving rise to many differing bonds, and so many different stretching frequencies. Figures 12-8, 12-9 (SLIDES) Ch12 IR and MS Page9